1. Field of the Invention
This invention relates generally to surgical methods and apparatus for addressing cardiomyopathy, and more specifically to methods and apparatus for restoring the architecture and normal function of a mammalian heart.
2. Description of the Related Art
The function of a heart in an animal is primarily to deliver life-supporting oxygenated blood to tissue throughout the body. This function is accomplished in four stages, each relating to a particular chamber of the heart. Initially deoxygenated blood is received in the right auricle of the heart. This deoxygenated blood is pumped by the right ventricle of the heart to the lungs where the blood is oxygenated. The oxygenated blood is initially received in the left auricle of the heart and ultimately pumped by the left ventricle of the heart throughout the body. It can be seen that the left ventricular chamber of the heart is of particular importance in this process as it is relied upon to pump the oxygenated blood initially through an aortic valve into and ultimately throughout the entire vascular system.
The amount of blood pumped from the left ventricle divided by the amount of blood available to be pumped is referred to as the ejection fraction of the heart. Generally, a healthier heart has a higher ejection fraction. A normal heart, for example may have a total volume of one hundred milliliters and an ejection fraction of 60 percent. Under these circumstances, 60 milliliters of blood are pumped with each beat of the heart. It is this volume in the normal heart of this example that is pumped with each beat to provide nutrients including oxygen to the muscles and other tissues of the body.
The heart is part of the body tissue, and the heart muscle also requires oxygenated blood. Its normal function is greatly upset by clotting or closure of the coronary arteries. When the coronary arteries are blocked, an associate portion of the heart muscle becomes oxygen-starved and begins to die. This is clinically referred to as a heart attack. Ischemic cardiomyopathy typically occurs as the rest of the heart dilates in an attempt to maintain the heart's output to the body.
As the ischemia progresses through its various stages, the affected myocardium dies losing its ability to contribute to the pumping action of the heart. The ischemic muscle is no longer capable of contracting so it cannot contribute to either squeezing or twisting motion required to pump blood. This non-contracting tissue is said to be “akinetic.” In severe cases the akinetic tissue, which is not capable of contracting, is elastic so that blood pressure tends to develop a bulge or expansion of the chamber. In this situation, this muscle tissue is not only akinetic, in that it does not contribute to the pumping function, but it is in fact “dyskinetic,” in that it detracts from the pumping function. This situation is particularly detrimental as the heart loses even more of its energy due to pumping the blood to the bulge instead of through the aorta.
After a heart attack, the body seems to realize that with a reduced pumping capacity, the ejection fraction of the heart is automatically reduced. For example, the ejection fraction may drop from a normal 60 percent to 20 percent. Realizing that the body still requires the same volume of blood for oxygen and nutrition, the body causes its heart to dilate or enlarge in size so that the smaller ejection fraction pumps about the same amount of blood. As noted, a normal heart with a blood capacity of seventy milliliters and an ejection fraction of 60 percent would pump approximately 42 milliliters per beat. The body seems to appreciate that this same volume per beat can be maintained by an ejection fraction of only 30 percent if the ventricle enlarges to a capacity of 140 milliliters. This increase in volume, commonly referred to as “remodeling”, not only changes the volume of the left ventricle, but also its shape. The heart becomes greatly enlarged. An enlarged heart will tend to change its architecture from the normal conical or apical shape, to a generally spherical shape.
On the level of the muscle fibers, it has been noted that enlargement or dilation of the heart causes the fibers to reorient themselves so that they are directed away from the inner heart chamber containing the blood. As a consequence, the fibers are poorly oriented to accomplish even the squeezing action, as the lines of force become less perpendicular to the heart wall. This change in fiber orientation occurs as the heart dilates and moves from its normal elliptical shape to its dilated spherical shape. The spherical shape further reduces pumping efficiency since the fibers which normally encircle the apex to facilitate writhing are changed to a more flattened formation as a result of these spherical configurations. The resulting orientation of these fibers produces lines of force, which are also directed laterally of the ventricle chamber. Thus, the dilation and resulting spherical configuration greatly reduces contraction efficiency.
Perhaps the most notable symptom of ischemic cardiomyopathy is the reduction in the ejection fraction which may diminish, for example, from a normal 60 percent to only 20 percent. This results clinically in fatigue and in an inability to do stressful activities that require an increase in output of blood from the heart. The output of blood by the enlarged heart at rest is kept normal, but the capacity to increase output of blood during stress (i.e., exercise, walking) is significantly reduced. Of course, the change in architecture has a dramatic effect on wall thickness, radius, and stress on the heart wall. In particular, it will be noted that absent the normal conical shape, the twisting motion of the heart, which can account for as much as one half of the pumping action, is lost. As a consequence, the more spherical architecture must rely almost totally on the lateral squeezing action to pump blood. This lateral squeezing action is inefficient and very different from the more efficient twisting action of the heart. The change in architecture of the heart will also typically change the structure and ability of the mitral valve to perform its function in the pumping process. Valvular insufficiency can also occur due to dilatation.
Although the dilated heart may be capable of sustaining life, it is significantly stressed and rapidly approaches a stage where it can no longer pump blood effectively. In this stage, commonly referred to as congestive heart failure, the heart becomes distended and is generally incapable of pumping blood returning from the lungs. This further results in lung congestion and fatigue. Congestive heart failure is a major cause of death and disability in the United States with approximately 400,000 new cases annually.
Following coronary occlusion, successful acute reprefusion by thrombolysis, (clot dissolution) percutaneous angioplasty, or urgent surgery can decrease early mortality by reducing arrhythmias and cardiogenic shock. It is also known that addressing ischemic cardiomyopathy in the acute phase, for example with reperfusion, may salvage the epicardial surface. Although the myocardium may be rendered akinetic, at least it is not dyskinetic. Post-infarction surgical re-vascularation can be directed at remote viable muscle to reduce ischemia. However, it does not address the anatomical consequences of the akinetic region of the heart that is scarred. Despite these techniques for monitoring ischemia, cardiac dilation and subsequent heart failure continue to occur in approximately 50 percent of post-infraction patients discharged from the hospital.
Various surgical approaches have been tried to treat the dilation of the ventricle by primarily reducing the ventricular volume. Some of these procedures involve removing or excluding dyskinetic and akinetic regions of the heart, then surgically joining the viable portions of the myocardial walls, typically with the use of a patch surgically placed in the walls using a Fontan stitch.
Typically, the exact placement of the patch has been visually determined using only a visual indication where the typically white scar tissue meets the typically red normal tissue. Location of the patch has been facilitated in a further procedure where a continuous suture has been placed around the ventricular wall to define a neck for receiving the patch. The neck has been formed in the white scar tissue rather than the soft viable muscle. This procedure has relied on cardioplegia methods to stop the beating of the heart and to aid in suture placement.
These surgical procedures have been met with some success as the ejection fraction has been increased, for example, from 24 percent to 42 percent. However, despite this level of success, it is often difficult for the surgeon to reconstruct the shape and size of the left ventricle. If the reconstructed ventricle is too small, the patient will not be able to pump enough oxygenated blood. If the reconstructed ventricle is too large, the ejection fraction may diminish. In addition to the size, the shape of the reconstructed ventricle is also important. If the left ventricle is reconstructed in a spherical shape, a twisting motion of the heart about its apex, which can account for as much as one half of the pumping action, is lost. As a consequence, the spherical shaped reconstructed ventricle must rely almost totally on the lateral squeezing action to pump blood. This lateral squeezing action is inefficient and very different from the more efficient twisting action of the heart. What is needed, therefore is a reliable method and apparatus to allow a surgeon to reconstruct the left ventricle to the appropriate shape, size and contour.